With the recent rapid advancement of portable and cordless electronic equipment, there are growing demands for downsized and lightweight nonaqueous electrolyte secondary batteries having larger energy density as a power supply for driving such equipment. In addition, technical development has been accelerating not only in consumer applications but also in large secondary batteries for power storage systems and electric vehicles that require long-term durability and safety. From these viewpoints, there are growing expectations for a nonaqueous electrolyte secondary battery having higher voltage and larger energy density, particularly for a lithium secondary battery, as a power supply for electronic equipment, a power storage system, or an electric vehicle.
A nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a separator disposed therebetween, and a nonaqueous electrolyte. The separator is made primarily of a polyolefin microporous membrane. For the nonaqueous electrolyte, a liquid nonaqueous electrolyte (nonaqueous electrolyte solution) containing a lithium salt, e.g. LiBF4 and LiPL6, dissolved in an aprotic organic solvent is used. For a positive electrode active material, a lithium cobalt oxide (e.g. LiCoO2) that has high potential to lithium and excellent safety, and is relatively easily synthesized is used. For a negative electrode active material, various carbon materials, such as graphite, are used. A nonaqueous electrolyte secondary battery structured as above is put to practical use.
Theoretically, graphite to be used as the negative electrode active material is capable of storing one lithium atom per six carbon atoms. Thus, the theoretical capacity density of the graphite is 372 mAh/g. However, a loss of capacity due to irreversible capacity decreases the actual discharge capacity density to approximately 310-330 mAh/g. For this reason, it is basically difficult to obtain a carbon material capable of storing and releasing lithium ions at a capacity density equal to or higher than this range.
Thus, in the demand for batteries having a much larger energy density, silicon (Si), tin (Sn), and germanium (Ge) alloyable with lithium, and oxides and alloys thereof are expected as a negative electrode active material having a large theoretical capacity density. Among these materials, particularly inexpensive Si and oxides thereof are widely studied.
However, when Si, Sn, and Ge, the oxides and alloys thereof storing lithium ions, the crystal structures thereof change and the volumes are increased. When the negative electrode active material greatly expands during charging, contact failure caused between the negative electrode active material and the current collector shortens the charge/discharge cycle life. To address this problem, the following proposals are made.
For example, a method of forming a thin film of negative electrode active material on a current collector is proposed to improve the contact failure between the negative electrode active material and the current collector caused by expansion (for example in Patent Document 1). Further, a method of forming an inclined columnar negative electrode active material on the surfaces of a current collector is proposed (for example in Patent Document 2). According to these proposals, the strong metallic bond between the negative electrode active material and the current collector can ensure stable current collection. Particularly for the latter proposal, spaces necessary and sufficient to accommodate the expansion are provided around the columnar active material. This structure prevents breakage of the negative electrode caused by expansion and shrinkage of the negative electrode active material and reduces the pressure and stress on the separator in contact therewith and positive electrode. Thus, the charge/discharge characteristics can particularly be improved.
However, when a silicon oxide (SiOx (0<x<2)) having excellent charge/discharge cycle characteristics is used for the negative electrode active material, the irreversible capacity of the silicon oxide is extremely large. For this reason, when the silicon oxide is simply combined with the positive electrode, a large portion of the reversible capacity of the positive electrode is wasted as irreversible capacity. Thus, compensation of lithium is necessary for achieving a large-capacity battery using a silicon oxide as a negative electrode active material thereof.
For this purpose, a large number of means for imparting metallic lithium to the negative electrode so that lithium is stored therein by a solid phase reaction are proposed, as the lithium compensation means. For example, a method including a step of evaporating lithium onto the surface of the negative electrode and a step of storing the electrode is proposed (for example in Patent Document 3).
However, when a negative electrode active material is formed by the methods disclosed in Patent Document 1 or 2, and lithium is evaporated onto the negative electrode surface as disclosed in Patent Document 3, lithium is not promptly absorbed in the negative electrode active material, and is non-uniformly deposited on the negative electrode surface. Such deposited lithium adheres to carrier rollers or the other side of the negative electrode, when the negative electrode is fed. This is because lithium is not diffused or stored in the negative electrode active material by a solid phase reaction and is in a state that it floats on the negative electrode surface. This adherence decreases productivity. Further, non-uniform storage of lithium in the negative electrode active material causes uneven expansion of the negative electrode active material resulting from charging, thus producing asperities on the negative electrode. This phenomenon causes non-uniform charge/discharge reaction and results in decreases in cycle characteristics, for example.    Patent Document 1: Japanese Patent Unexamined Publication No. 2002-83594    Patent Document 2: Japanese Patent Unexamined Publication No. 2005-196970    Patent Document 3: Japanese Patent Unexamined Publication No. 2005-38720